The structure and function of the molybdenum of molybdenum enzymes (xanthine oxidase, sulfite oxidase, nitrate reductase, nitrogenase) will be explored by a study of model Mo(VI), (V), (IV), and (III) complexes. A variety of complexes with ligands of biochemical interest will be synthesized, with the emphasis on those containing thiol groups. Complexes will be characterized by analysis, electronic, nmr, IR and esr spectrometry, cyclic voltammetry and coulometry. Where possible, X-ray crystallographic structures will be obtained. Reduction potentials and esr parameters (Mo(V) monomers) will be determined. Reactions with substrates of biochemical significance (SO3, purines, aldehydes, NO3, flavins, heme and nonheme iron complexes) will be investigated kinetically; rate constants and activation parameters will be obtained. The data will be used to interpret the corresponding properties of the Mo enzymes, allowing conclusions concerning the structure of the Mo centers and the role of the metal in the catalytic reactions to be made.